1. Field of the Invention
The present invention relates to an automatic iris control mechanism for a solid-state imaging apparatus using a charge-coupled imaging device (hereinafter referred to as "CCD imaging device").
2. Description of the Prior Art
Automatic iris control mechanisms for solid-state imaging apparatus, have conventionally controlled the intensity of light transmitted through the iris to the imaging device. This intensity is controlled at a proper level in accordance with the level of a video signal obtained as a result of the imaging operation.
A solid-state imaging apparatus using the CCD imaging device can also employ the same automatic iris control mechanism, by which the intensity of light passing through the iris and falling upon the CCD imaging device is controlled in accordance with the video signal level. The CCD imaging device, employed in the solid-state imaging apparatus, generally eliminates the influence from the fluctuations in a dark current due to temperature change. To do this, the additional imaging area is formed with an area, which is shielded from an incident light in order to supply an electrical signal corresponding to an optical black. The output signal corresponding to the optical black is also used as the reference signal for the imaging signal, from which the video signal level is detected to control the intensity of incident light. In CCD imaging devices, having none of the afore-mentioned additional imaging area, the horizontal transfer register is subjected to the idle transfer, after the charges in one horizontal line are transferred. The output signal resulting from that idle transfer is used as the reference signal of the imaging signal.
As is well known in the art, the CCD imaging device is divided into two types, i.e., the interline transfer type and the frame transfer type. The interline transfer type device is composed of many light sensors, vertical transfer registers and a horizontal transfer register. The frame transfer type device is composed of an imaging area, a storage area and a horizontal transfer register. Generally speaking, when an intense light enters the light sensors or the imaging area the charge generated by that incident light may exceed the upper level that can be accumulated in the potential wells of the light sensors or the imaging area. The charge will then overflow from the potential wells and pour into the neighboring potential well to deteriorate the reproduced image. Moreover, if the charge is extremely excessive, the overflow charges reach the horizontal transfer area and fill up the potential wells thereof. Under this particular condition, neither a video signal nor a reference signal can be obtained. All of the output levels become equal so that the video signal level can no longer be detected by the afore-mentioned method for detecting the video signal level. The result of such a situation is equivalent to that in which no light comes from an object. In both situations, the maloperation of the automatic iris control mechanism operates the iris in the direction which increases the light intensity incident upon the CCD imaging device, despite an already intense light.